Heterogeneous photoconductor composition formed by low-temperature conditioning

ABSTRACT

HETEROGENEOUS PHOTOCONDUCTIVE COMPOSITIONS ARE FORMED BY FIRST DISSOLVING A DYE IN A SOLVENT. AFTER SUBSTANTIAL DISSOLUTION OF THE DYE, POLYMER AND PHOTOCONDUCTOR ARE ADDED TO THE DYE SOLUTION TO FORM A COATING DOPE. THE RESULTANT DOPE IS SUBJECTED TO LOW-TEMPERATURE CONDITIONING BY COOLING TO A REDUCED TEMPERATURE AND HOLDING AT THAT TEMPERATURE FOR A PERIOD OF TIME. THE CONDITIONED DOPE IS THEN WARMED TO COATING TEMPERATURE, COATED AS A THIN FILM AND ALLOWED TO DRY TO FORM A HETEROGENEOUS COMPOSTION CONTAINING SUBMICRON SIZED PARTICLES OF A CO-CRYSTALLINE COMPLEX. ELECTROPHOTOGRAPHIC ELEMENTS ARE FORMED BY COATING THIS DOPE ON A SUPPORT.

Patent Oflice 3,679,406 Patented July 25, 1972 3,679,406 HETEROGENEOUS PHOTOCONDUCTOR COM- POSITION FORMED BY LOW-TEMPERA- TURE CONDITIONING Frederick J. Kryman, Batavia, N.Y., assignor to Eastman Kodak Company, Rochester, N.Y. No Drawing. Filed Nov. 13, 1970, Ser. No. 89,446 Int. Cl. G03g 5/00; H011 13/00 U.S. C]. 96-16 Claims ABSTRACT OF THE DISCLOSURE Heterogeneous photoconductive compositions are formed by first dissolving a dye in a solvent. After substantial dissolution of the dye, polymer and photoconductor are added to the dye solution to form a coating dope. The resultant dope is subjected to low-temperature conditioning by cooling to a reduced temperature and holding at that temperature for a period of time. The conditioned dope is then warmed to coating temperature, coated as a thin film and allowed to dry to form a heterogeneous composition containing submicron sized particles of a co-crystalline complex. Electrophotographic elements are formed by coating this dope on a support.

This invention relates to electrophotography and to photoconductive elements and structures useful in electrophotography. In addition, this invention relates to methods for preparing electrophotographic elements.

'Electrophotographic imaging processes and techniques have been extensively described in both the patent and other literature. Generally, these processes have in common the steps of employing a normally insulating photoconductive element which is prepared to respond to imagewise exposure with electromagnetic radiations by forming a latent electrostatic charge image. A variety of subsequent operations, now well-known in the art, can then be employed to produce a permanent record of the image.

One type of photoconductive insulating structure of element particularly useful in electrophotography utilizes a composition containing a photoconductive insulating material dispersed in a resinous material. A unitary electrophotographic element is generally produced in a multilayer type of structure by coating a layer of the photoconductive composition onto a film support previously overcoated with a layer of conducting material or the photoconductive composition may be coated directly onto a conducting support of metal or other suitable conducting material. Such photoconductive compositions have shown improved speed and/ or spectral response, as well as other desired electrophotographic characteristics when one or more photosensitizing materials or addenda are incorporated into the photoconducting composition. Typical addenda of this latter type are disclosed in U.S. Pats. 3,250,615 by VanAllan et a1. issued May 10, 1966, 3,141,770 by Davis et al. issued July 21, 1964, and 2,987,395 by Jarvis issued June 6, 1961. Generally, photosensitizing addenda to photoconductive compositions are incorporated to effect a change in the sensitivity or speed of a particular photoconductor system and/or a change in its spectral response characteristics. Such addenda can enhance the sensitivity of an element to radiation at a particular wavelength or to a broad range of Wavelengths where desired. The mechanism of such sensitization is presently not fully understood. The phenomenon, however, is extremely useful.

Usually the desirability of a change in electrophotographic properties is dictated by the end use contemplated for the photoconductive element. For example, in document-copying applications, the spectral electrophotographic response of the photoconductor should be capable of reproducing the wide range of colors which are typically encountered in such use. If the response of the photoconductor falls short of these design criteria, it is highly desirable if the spectral response of the composition can be altered by the addition of photosensitizing addenda to the composition. Likewise, various applications specifically require other characteristics such as the ability of the element to accept a high surface potential, and exhibit a low dark decay of electrical charge. It is also desirable for the photoconductive element to exhibit high speed as measured in an electrical speed or characteristic curve, a low residual potential after exposure and resistance to fatigue. Sensitization of many photoconductive compositions by the addition of certain dyes selected from the large number of dyes presently known has hitherto been widely used to provide for the desired flexibility in the design of photoconductive elements in particular photoconductor-containing systems. Conventional dye addenda to photoconductor compositions have generally shown only a limited capability for over-all improvement in the totality of electrophotographic properties which cooperate to produce a useful electrophotographic element or structure. The art is still searching for improvements in shoulder and toe speeds, improved solid area reproduction characteristics, rapid recovery and useful electrophotographic speed from either positive or negative electrostatic charging.

A high-speed heterogeneous or aggregate photoconductive system was developed by VWlliam A. Light which overcomes many of the problems of the prior art. This aggregate composition is the subject matter of copending application Ser. No. 804,266 filed Mar. 4, 1969, now U.S. Pat. No. 3,615,414, and entitled Nover Photoconductive Compositions and Elements. The addenda disclosed therein are responsible for the exhibition of desirable electrophotographic properties in photoconductive elements prepared therewith. However, in accordance with the procedures described therein, the preparation of electrophotographic elements uses a solvent treatment step subsequent to the coating step. In an, effort to avoid this secondary treatment step, a novel method of preparation of photoconductive compositions of the type described by Light is disclosed in copending Eugene P. Gramza application Ser. No. 821,513 filed May 2, 1969, now U.S. Pat. No. 3,615,415, and entitled Method for the Preparation of Photoconductive Compositions. This latter method involves the high-speed shearing of the photoconductive composition prior to coating and thus eliminates subsequent solvent treatment.

An additional problem encountered in forming such heterogeneous photoconductive compositions is that many of the dyes useful in preparing such compositions have several crystalline structures. Depending upon which crystalline structure of the dye is present when using the above techniques, the formation of the aggregate compositions can be relatively easy or quite difficult. In an effort to avoid the difficulties often encountered with different crystalline structures, a novel dye-first procedure is used, as described in copending Eugene P. Gramza et al. application Ser. No. 816,831, now U.S. Pat. No. 3,615,396, filed Apr. 16, 1969, and entitled Method for the Preparation of Photoconductive Compositions.

The so-called dye-first technique is extremely useful in that it is simple and efiicient; however, it results in the formation of aggregates of a larger size. In general, aggregates formed by the dye-first method are larger than about 1 micron. These larger-size aggregates are suitable for many uses; however, for certain applications, it is highly desirable to have considerably smaller-sized aggregates. Accordingly, there is a need for a simple, eflicient method of obtaining aggregate photoconductive 3 compositions containing small-sized aggregates and which process does not require large amounts of energy.

It is therefore, an object of this invention to pro- .vide the art of electrography with a novel method ofv preparing aggregate photoconductive compositions.

- It is an additional object of this invention to provide a novel method of preparing heterogeneous photoconductive compositions containing a discontinuous phase comprisedof submicron-sized aggregates.

It is a further object to provide a novel method paring sensitized electrophotographic elements.

- It has been discovered that, when the heterogeneous or aggregate photoconductive compositions of William A. Light are prepared in a prescribed manner, formation of the heterogeneous composition is obtained without the necessity of any secondary treatment or additional overcoating steps. In particular, when the coating dope is subjected to low-temperature conditioning prior to coating, improved control of aggregate size and uniformity is obtained.

The method of this invention is used to form heterogeneous multiphase photoconductive compositions comprised of an organic sensitizing dye and an electrically insulating, film-forming polymeric material. The present method is relatively uncomplicated and provides results which are readily reproducible. One of the essential features of the instant invention is the low-temperature conditioning of the dope prior to coating and drying. After conditioning at reduced temperatures, the dope is equilibrated to a warmer temperature and coated onto a suitable support which results in the formation of a of preseparately identifiable multiphase composition. The heterogeneous nature of the resultant composition is generally apparent when viewed under magnification, although such compositions may appear to be substantially optically clear to the .naked eye. Suitably, the dye-containing aggregate in the discontinuous phase is submicron in size and is predominantly in the size range of about 0.01 to about 0.75 micron. However, it should be noted that when "the heterogeneous compositions prepared by this invention are used in conjunction with a particulate photoconductor such as zinc oxide, another discontinuous phase will be present which generally will not fall within this size range.

Typically, the heterogeneouscompositions formed by the present method are multiphase organic solids. The

I polymeric material or vehicle comprises an amorphous matrix or continuous phase which contains a discrete discontinuous phase as distinguished from a solution. The discontinuous phase is the aggregate species which is a co-crystalline complex comprised of dye and polymer.

The term co-crystalline complex as used herein has reference to a crystalline compound which contains dye and polymer molecules co-crystallized in a single crystalline structure to form a regular array of the molecules in a three dimensional pattern.

When the present compositions are used in conjunction with an organic photoconductor, the resultant photo- 'conductive composition generally contains only two phases Of course, the present multiphase compositions may also contain additional discontinuous phases of trapped impurities, etc. Another feature characteristic of the heterogeneous compositions formed in accordance with this invention is that the wavelength of the radiation-absorption maximum characteristic of such compositions is substantially shifted from the wavelength of the radiation- ,absorption maximum of a substantially homogeneous dye- 4 polymer solid solution formed of similar constituents. The new absorption maximum characteristic of the aggregates formed by this method is not necessarily an over-all maximum for this system, as this will depend upon the relative amount of dye in the aggregate. Such an absorption maximum shift in the formation of multiphase heterogeneous systems for the present invention is generally of the magnitude of at least about 10 nm. If mixtures of dyes are used, one dye may cause an absorption maximum shift to a long wavelength and another dye cause an absorption maximum to a shorter wavelength. In such cases, a formation of the heterogeneous compositions can more easily be identified by viewing under magnfication.

Sensitizing dyes and electrically insulating polymeric materials are used in forming these heterogeneous compositions. Typically, pyrylium dyes, including pyrylium, thiapyrylium and selenapyrylium dye salts are useful in forming such compositions. Such dyes include those I which can be represented by the following formula:

wherein R R R, R and R can each represent (a) a hydrogen atom; (b) an alkyl group typically having from 1 to 15 carbon atoms, such as methyl, ethyl, propyl, isopropyl, butyl, tertiary butyl, amyl, isomyl, hexyl, octyl, nonyl, dodecyl, etc., (c) alkoxy groups like methoxy, ethoxy, propoxy, butoxy, amyloxy, hexoxy, octoxy and the like; and (d) aryl groups including substituted aryl groups such as phenyl, 4-diphenyl, alkylphenyls as 4-ethylphenyl, 4-propylphenyl and the like, alkoxyphenyls as 4-ethoxyphenyl, 4-methoxyphenyl, 4-amyloxyphenyl, 2-hexoxy-- phenyl, 2-methoxyphenyl, 3,4-dimethoxyphenyl and the like, fi-hydroxy al-koxyphenyl as 2-hydroxyethoxyphenyl, 3-hydroxyethoxyphenyl and the like, 4-hydroxyphenyl, halophenyl as 2,4-dichlorophenyl, 3,4-dibromophenyl, 4- chlorophenyl, 3,4-dichlorophenyl and the like, azidophenyl, nitrophenyl, aminophenyls as 4-diethylaminophenyl, 4-dianethylaminopheny1 and the like, naphthyl; and vinyl substituted aryl groups such as styryl, methoxystyryl, diethoxystyryl, dimethylaminostyryl, 1-butyl-4-pdimethylaminophenyl-1,3-butadienyl, B-ethyl-4 dimethylaminostyryl and'the like; and where X is a sulfur, oxygen or selenium atom, and Z- is an anionic function, including such anions as perchlorate, fluoroborate, iodide, chloride, bromide, sulfate,-periodate, p-toluenesulfonate, hexafluorophosphate and the like. In addition, the pair R and R as well as the pair R and R can together be the necessary atoms to complete an aryl ring fused to the pyrylium nucleus. Typical'members of such pyrylium dyes are listed in Table l.

phenylthiapyrylium perchlorate.

propyl, butyl, tertiary butyl, pentyl, hexyl, heptyl, octyl,

nonyl, decyl and the like including substituted alkyl radi- "cals such as trifluoromethyl, etc., and an aryl radical such as phenyl and naphthyl, including substituted aryl radicals having such substituents as a halogen, alkyl radicals of from 1- to about carbon atoms, etc.;and R and R when taken together, can represent the carbon atoms necessary to form a cyclic hydrocarbon radical including cycloalkanes such as cyclohexyl and polycycloalkanes such as norbomyl, the total number of carbon atoms in 'R; and R being up to 19; R5 and R, can each be hydrogen, an alkyl radical of from 1 to about 5 carbon atoms, e.g., methyl, ethyl, isopropyl, hexyl, amyl, etc., or a halogen atom such as chloro, bromo, iodo, etc.; and R is a divalent radical selected from the following:

Preferred polymers useful in the present methodof forming aggregate crystals are hydrophobic carbonate polymers comprised of the following recurring unit:

lowing U.S. patents: 2,999,750 by Miller et al. issued Sept. 12, 1961; 3,038,874 by Laakso et al. issued June 12,

1962; 3,038,879 by Laakso et al. issued June 12, 1962;

3,038,880 by Laakso et al. issued June 12, 1962;

03,106,544 by Laakso et al. issued Oct. 8, 1963; 3,106,545 by Laakso et al. issued Oct. 8, 1963; and 3,106,546 by Laakso et al. issued Oct. 8, 1963. A wide range of filmforming polycarbonate resins are useful, with completely satisfactory results being obtained when using commercial polymeric materials which are characterized by an inherent viscosity of about 0.5 to 0.6. In addition, a'high weight Bisphenol A polycarbonate can be very useful. Preferably, such high molecular weight materials have an inherent viscosity of greater than about 1 as measured in 1,2-dichl0roethane at a concentration of 0.25 g./ 100 ml. and a temperature of about 25 C. The use of high molecular weight polycarbonate, for example, facilitates molecular weight material such as a high molecular "the formation of aggregate compositions having increased speeds.

The following polymers are included among the mate rials useful in the practice of this invention.

TABLE 2 Number Polymeric material 1 Poly(4,4-is0propylidenediphenylene-co-L t'cyclohexyldimethylcarbonate) 2 Poly(3,Elethylenedioxyphenylene thlocarbonate).

3 Poly(4,4-isopropylidenediphenylene carbonate-coterephthalate) 4 Poly(4,4-isopropylidenedlphenylene carbonate).

5. Poly (4,4-lsopropylidenediphenylene thlocarbonate) 6. Poly(2,2-butanebis-4-phenylene carbonate).

7- Poly(4,4-isopropylidenediphenylene carbonate-blockethylene oxide).

8 P01y(4,4-isopropylidenediphenylene carbonate-blocktetramethyleneoxide) 9 Poly[4,4-isopropylidenebis(Z-methylphenylene) carbonate].

10 Poly(4,4'-isopropylidenediphcnylenc-co-l,4-phenylenc carbonat ll Poly(4,4-isopropylidenedlphenylene-co-l,3-phenylenc carbonat l2 Poly(4,4-isopropylidenediphenyIene-eMAdlphenylene carbonate).

13 Poly(4,4'-isopropylidenediphenylene-co-4,4-oxydiphenylene carbonate).

14 Poly(4,4-isopropylidenediphenylene-co-4,4-carbonyldlphenylene carbonate).

15 Poly(4,4!-isopropylidenediphenylene-co-4,4-ethylenedlphenylene carbonat 16 Poly[4,4'-methylenebis 2-methylphenylene)carbonate].

17. olyll,1-(p-bromophenylethane)bis(4-phenylene) carbonate].

18 Poly[4,4-isopropylldenedlphenylcne-co-sultonylbis(4- phenyl) carbonate].

19 Polyll,l-cyclohexanebis(t-phenyl) carbonate].

20 Poly[4,4'-isopropylidene bis(Z-chlorophenyDcarbonate].

21 Poly(hexatluoroisopropylidene-dl-t-phenyl carbonate);

22 Poly(4,4-isopr0pylldenediphenyl-4,4'-isopropylidene dibenzoate) 23 Poly(4,4-isopropylidenedibenzyl-4,4-lsopropylldene dibenzoate) 24 Poly[2,243-methylbutane)b1s-4-phenyl carbonate].

Poly[2,2-E3,3-dimethylbutane)bis+phenyl carbonate].

Poly{1,1- 1-(1-naphthyl)]bis-4-phenyl carbonate] Poly[2,2-(4-methylpentane)bis-4-phenyl carbonate].

Poly[4,4-(2-norbornylidene)diphenyl carbonate].

Poly[4,4-(hexahydro-4,7-methanolndan-fi-ylidene) diphenyl carbonate].

Sensitized compositions formed according to the present inention can readily be used for enhancing the sensitivity and extending the spectral range of sensitivity of a variety of organic photoconductors and inorganic photoconductors including both nand ptype photoconductors. A typical example of an inorganic photoconductor would. be zinc oxide. The present invention can be used in connection with many organic, including organo-metallic, photoconducting materials which have little or substantially no persistence of photoconductivity. Representative organo-metallic compounds are the organic derivatives of Groups IIIa, Na, and Va metals such as those having at least one amino-aryl group attached to the metal atom. Exemplary organometallic compounds are the triphenylp-dialkylaminophenyl derivatives of silicon, germanium, tin and lead, and the tri-p-dialkylaminophenyl derivatives of arsenic, antimony, phosphorus, bismuth boron, aluminum, gallium, thallium and indium. Useful photoconductors of this type are described in copending U.S. patent application Ser. No. 650,664 by Goldman and Johnson filed July 3, 1967, and 755,711 by Johnson filed Aug. 27, 1968, now U.S. Pat. No. 3,607,257. 7

An especially useful class of organic photoconductors is referred to herein as organic amine photoconductors. Such organic photoconductors have as a common, structural feature at least one amino group. Useful organic photoconductors which can be spectrally sensitized in accordance with this invention include, therefore, arylamine, compounds comprising (1) diarylamines such as diphenylamine, dinaphthylamine, N,N' diphenylbenzidine, N1

phenyl l naphthylamine, N phenyl 2 e naphthylamine, N,N diphenyl p phenylenediamine, 2 carthose described in Fox, U.S. Patent 3,240,597 issued Mar.

15, 1966, and the like, and (2) triarylamines including (a) nonpolymeric triarylamines such as triphenylamine, N,N,N',N' tetraphenyl m phenylenediamine, 4-acetyltriphenylamine, 4 hexanoyltriphenylamine, 4-lauroyltriphenylamine, 4 hexyltriphenylamine, 4 dodecyltriphenylamine, 4,4 bis(diphenylamino)benzil, 4,4 bis- (diphenylamino)benzophenone and the like, and (b) polymeric triarylamines such as poly[N,4-(N,N',N'-triphenylbenzidine)], polyadipyltriphenylamine, polysebacyltriphenylamine, polydecamethylenetriphenylamine, poly N (4 vinylphenyl)diphenylamine, poly N- (vinylphenyl) u,a dinaphthylamine and the like. Other useful amine-type photoconductors are disclosed in US. Patent 3,180,730 issued Apr. 27, 1965.

Useful photoconductive substances capable of being sensitized in accordance with this invention are disclosed in Fox, US. Patent 3,265,496 issued Aug. 9, 1966, and include those represented by the following general formula:

wherein T represents a mononuclear or polynuclear divalent aromatic radical, either fused or linear (e.g., phenyl, naphthyl, biphenyl, binaphthyl, etc.), or a substituted divalent aromatic radical of these types wherein said substituent can comprise a member such as an acyl group having from 1 to about 6 carbon atoms (e.g., acetyl, pripionyl, butyryl, etc.), an alkyl group having from 1 to about 6 carbon atoms (e.g., methyl, ethyl, propyl, butyl, etc.), an alkoxy group having from 1 to about 6 carbon atoms (e.g., methoxy, ethoxy, propoxy, pentoxy, etc.), or a nitro group; M represents a mononuclear or polynuclear monovalent aromatic radical, either fused or linear (e.g., phenyl, naphthyl, biphenyl, etc.), or a substituted monovalent aromatic radical wherein said substituent can comprise a member, such as an acyl group having from 1 to about 6 carbon atoms (e.g., acetyl, propionyl, butyryl, etc.), an alkyl group having from 1 to about 6 carbon atoms (c.g., methyl, ethyl, propyl, butyl, etc.), an alkoxy group having from 1 to about 6 carbon atoms (e.g., methoxy, propoxy, pentoxy, etc.), or a nitro group; Q can represent a hydrogen atom, a halogen atom or an aromatic amino group, such as MNH-; b represents an integer from 1 to about 12; and R represents a hydrogen atom, a mononuclear or polynuclear aromatic radical, either fused or linear (e.g., phenyl, naphthyl, biphenyl, etc.), a substituted aromatic radical wherein said substituent comprises an alkyl group, an alkoxy group, an acyl group, or a nitro group, or a poly(4-vinylphenyl) group which is bonded to the nitrogen atom by a carbon atom of the phenyl group.

Polyarylalkane photoconductors are particularly useful in producing the present invention. Such photoconductors are described in US. Patent 3,274,000 by Noe et a1. issued Sept. 20, 1966, French Patent 1,383,461, and in copending application of Seus and Goldman entitled Photoconductive Elements Containing Organic Photoconductors, Ser. No. 627,857 filed Apr. 3, 1967. These photoconductors include leuco bases of diaryl or triaryl methane dye salts, 1,1,1-triarylalkanes wherein the alkane moiety has at least two carbon atoms and tetraarylmethanes, there being substituted an amine group on at least one of the aryl groups attached to the alkane and methane moieties of the latter two classes of photoconductors which are non-leuco base materials.

Preferred polyarylalkane photoconductors can be represented by the formula:

wherein each of D, E and G is an aryl group and I is a hydrogen atom, an alkyl group or an aryl group, at least one of D, E and G containing an amino substituent. The aryl groups attached to the central carbon atom are preferably phenyl groups, although naphthyl groups can also be used. Such aryl groups can contain such substituents as alkyl and alkoxy typically having 1 to 8 carbon atoms, hydroxy, halogen, etc. in the ortho, meta or para positions, ortho-substituted phenyl being preferred. The aryl groups can also be joined together or cyclized to form a fiuorene moiety, for example, The amino sub stituent can be represented by the formula:

TABLE 3 Compound number Name of compound 1 4, 4-benzylidene bls(N, N- liethyl-n1-tolui line).

2 4, 4-di3xnlna-4 iirnethylaminyf, 2-dimethyltriphenyl methane.

3 4, 4-bis(diethylamino)-2, fi-diehloro-Z', 2-dirnethyltrl phenylmethane.

4 4, 4"-bis(diethylamiuo) -2, 2"-din1ethyldiphenylnaphthylmethane:

5 2, 2- limethyl-4, 4', 4-trls(dimethylamlnu) triphen ylmethane.

G 4, 4-bis(diethylarniuo) -4- 1imetl1ylamiuo-2, 2-dimethyltriphenylmothane.

7 4 4-bis(diethylamir10)-2-chloro-2' 2-dimethyl-4- (limethylaminotriphenylmethane.

8 4', 4"-bis(diethylamino) 4:limethylamino-2, 2', 2-trimethyltrlphenylmethane.

0 4, 4bis(dirnethylarnino)-2chloro2', 2-dimethyltriphenylmethane.

10 4, 4bis(rlimethylarm'no) -2, 2 -dimethyl-4-n1ethoxy triphenylmethane.

Bis(4-diethylamino) -1, 1, l-triphenylethane.

Bis(4-diethylamino)tetraphenylmethane.

4, 4"-bis(benzylethylamino) -2, 2-dimethyltriphcnylmethane. 14 4, 4-bis(diethylamino)-2, 2-diethoxytriphenylmethane. 4, 4bis(dimethylainino) -1, 1, l-triphenyleth ane.

l-(4-N, N -dimethylaminophenyl) -1, l-dipheuyle-thaue. 4-dimethylaminotetrapheuylmethane. 4-diethylaminotctraphenylmethane.

Another class of photoconductors useful in this invention are the 4 diarylamino-substituted chalcones. Typical compounds of this type are low-molecular-weight nonpolymeric ketones having the general formula:

0 3 11:6 H il-R2 wherein R and R are each phenyl radicals including substituted phenyl radicals and particularly when R is a phenyl radical having the formula:

where R, and R are each aryl radicals, aliphatic residues of 1 to 12 carbon atoms such as alkyl radicals preferably having 1 to 4 carbon atoms or hydrogen. Particularly advantageous results are obtained when R is a phenyl radical including substituted phenyl radicals and where R is diphenylaminophenyl, dimethylaminophenyl or phenyl.

Other photoconductors which can be used with the present aggregate compositions include rhodamine B, malachite green, crystal violet, phenosafranine, cadmium sulfide, cadmium selenide, parachloronil, benzil, trinitrofiuorenone, tetranitrofluorenone, etc.

' range for the, photoconductor is from about to about 80 weight percent. Of course, if it is desired to use the present aggregate compositions alone as the photoconductive substance, then no photoconductor would be added. In addition to the photoconductors described above, polymeric photoconductors, e.g., poly(N-vinyl carbazole) halogenated poly(N-vinyl carbazole), etc., can also be used, if desired.

According to the process of this invention, a pyrylium dye, as hereinabove defined, is mixed into a coating solvent and stirred for a period of time to insure complete dissolution of the dye. Generally, stirring for a period of up to about 2. to 3 hours is sufficient to obtain substantially complete dissolution of the dye. Typically, the dye is usually dissolved at about room temperature (about 25 C.). The time of stirring will vary depending upon the dye concentration and the total amount of solution being prepared. The concentration of dye which is first substantially completely dissolved in the coating solvent can be varied considerably, being limited, of course, by the solubility of a particular dye in a particular solvent and by the desired dye concentration in a given composition. Higher dye concentrations generally give rise to resultant photoconductive compositions of higher electrophotographic speeds. Useful results are obtained by using the described dyes in amounts of from about /2 to about 30% by weight of the total solids added to the coating composition.

After the dye is in solution, the polymeric binder is then added and the solution is stirred for a period of time, with stirring for about 2 to 3 hours generally being suflicient. Next, the photoconductor is added, if desired, and the combined solution is stirred for about 30 to 60 minutes. The organic coating solvents useful for preparing the above coating dopes can be selected from a variety of materials. Useful liquids are hydrocarbon solvents, in-

eluding substituted hydrocarbon solvents, with preferred materials being halogenated hydrocarbon solvents. The requisite properties of the solvent are that it be capable of dissolving the pyrylium dye and capable of dissolving or at least highly swelling or solubilizing the polymeric ingredient of the composition. In addition, it is helpful if the solvent is volatile, preferably having a boiling point of less than about 200 C. Particularly useful solvents include halogenated lower alkanes having from 1 to about 3 carbon atoms, such as dichloromethane, dichloroethane, dichloropropane, trichloromethane, trichloroethane, tribromomethane, trichloromonofluoromethane, trichlorotrifluoroethane, etc.; aromatic hydrocarbons such as benzene, toluene as well as halogenated benzene compounds such as chlorobenzene, bromobenzene, dichlorobenzene,

etc.; ketones such as dialkyl ketones having 1 to about 3 carbon atoms in the alkyl moiety, such as dimethylketone, methylethylketone, etc.; and ethers such as tetrahydrofuran, etc. Mixtures of these and other solvents can also be used.

After formation of the coating dope as described above, the dope is subjected to low-temperature conditioning. This involves cooling the dope to a temperature below room temperature and holdingx'the dope at this reduced temperature for a period of time. In general, the coating dope is cooled to a temperature below about 5 C. and held for a period of time which can vary from about 1 hour to about days, depending upon the temperature at which the dope is held. The temperature can beas low as the temperatureof liquid nitrogen (19*6 C.),'if desired. Typically, if the temperature is quite low, the time of holding can be reduced. Conversely, if the 12 temperature is closer to room temperature, it is usually desirable to maintain the coating dope at that temperature for a longer period of time. Preferably, the coating dope is maintained at a temperature below about 5 C. for a period of time of at least about 24 hours. The coating dope is stored at these low temperatures without agitation, and after low-temperature conditioning is complete, the composition is re-equilibrated at room temperature and stirred for a period of about 1 hour prior to coating. At this point, the photoconductor can be added, if desired, and if not added prior to the low-temperature conditioning. The dope is then coated on a suitable conducting support and allowed to dry. Drying can be accomplished by any convenient means such as simply allowing the solvent to evaporate at ambient conditions. The coating can also be dried by circulating air or gentle heating, with care being taken to avoid any heat damage. The aggregate composition is thus formed upon coating and drying of the dope without the need for any aftertreatment.

Coating thicknesses of the present photoconductive compositions on a support can vary widely. More generally, a coating in the range of about 10 microns to about 1250 microns before drying is useful for the practice of this invention. The preferred range of coating thickness is found to be in the range from about 50 microns to about 750 microns before drying, although useful results can be obtained outside of this range.

Suitable supporting materials for coating the photo conductive layers of the present invention can include any of a wide variety of electrically conducting supports, for example, paper (at a relative humidity above 20 percent); aluminum foil-paper laminates; metal foils such as aluminum foil, zinc foil, etc.; metal plates such as aluminum, copper, zinc, brass and galvanized plates; vapordeposited metal layers such as silver, nickel, aluminum and the like coated on paper or conventional photographic film bases such as cellulose acetate, polystyrene, poly(ethylene terephthalate), etc. Such conducting materials as nickel can be coated by vacuum deposition on transparent film supports in sufiiciently thin layers to allow electrophotographic elements prepared therewith to be exposed from either side of such elements. An especially useful conducting support can be prepared by coating a support material such as poly(ethylene terephthalate) with a conducting layer containing a semiconductor such as cuprous iodide dispersed in a resin or vacuumdeposited. Such conducting layers, both with and without insulating barrier layers are described in US. Patents 3,245,833 by Trevoy issued Apr. 12, 1966, and 3,428,451 by Trevoy issued Feb. 18, 1969. Likewise, a suitable conducting coating can be prepared from the sodium salt of a carboxyester lactone of m-aleic anhydride and a vinyl acetate polymer. Such kinds of conducting layers and methods for their optimum preparation and use are disclosed in US. Patents 3,007,901 by Minsk issued Nov. 7,

1961, and 3,267,807 by Sterman et al. issued July 26,

The present technique of substantially completely dissolving the sensitizing dye in a coating solvent prior to addition of polymeric binder and/or photoconductor, followed by low-temperature conditioning, gives rise to several advantages. It is possible to form aggregate photoconductive compositions having a high dye concentration in a single coating operation. The high dye concentration in the aggregate composition typically results in greater electrophotographic speeds. Furthermore, the present method makes it possible to produce readily highspeed photoconductive compositions in a manner which is independent of the crystalline structure of the dye or dyes used in preparing the compositions. Additionally, the present process has the advantage of forming subrnicronsized aggregate particles which are highly desirable for some applications. By virtue of the small aggregate size, the resultant photoconductive layers have a glossy appearance. This is a result of a very high level of specular reflection obtained with layers containing submicron aggregate particles.

In general, the elements produced by the techniques of this invention can be employed in any of the well-known electro-photographic processes which require photoconductive layers. One such process is the xerographic proc ess. In a process of this type, an electrophotographic element is held in the dark and given a blanket electrostatic charge by placing it under a corona discharge. This uniform charge is retained by the layer because of the substantial dark insulating property of the layer, i.e., the low conductivity of the layer in the dark.'The electrostatic charge formed on the surface of the photoconductive layer is then selectively dissipated from the surface of the layer by imagewise exposeure to light by means of a conventional exposure operation such as, for example,

by a contact-printing technique, or by lens projection of an image, and the like, to form thereby a latent electrostatic image in the photo-conductive layer. 'Exposing the surface in this manner forms a pattern of electrostatic charge by virtue of the fact that light energy striking the photoconductor causes the electrostatic charge in the light-struck areas to be conducted away from the surface in proportion to the intensity of the illumination in a particular area.

The charge pattern produced by exposure is then developed or transferred to another surface and developed there, i.e., either the charged or uncharged areas rendered visible, by treatment wtih a medium comprising electrostatically responsive particles having optical density. The developing electrostatically responsive particles can be in the form of a dust, i.e., powder, a pigment in a resinous carrier, i.e., toner. A preferred method of applying such toner to a latent electrostatic image for solid area development is by the use of a magnetic brush. Methods of forming and using a magnetic brush toner applicator are known in the art, e.g., US. Patents 2,786,- 439 by Young issued Mar. 26, 1957; 2,786,440 by Giaimo issued Mar. 26, 1957; 2,786,441 by Young issued Mar. 26, 1957; and 2,874,063 by Greig issued Feb. '17, 1959. Liquid development of the latent electrostatic image may also be used. In liquid development, the developing particles are carried to the image-bearing surface in an electrically insulating liquid carrier. Methods of development of this type are widely known and have been described in the patent literature, for example US. Patent 2,907,- 674 by Metcalfe et al. issued Oct. 6, 1959. In dry developing processes, the most widely used method of obtaining a permanent record is achieved by selecting a developing particle which has as one of its components a lowmelting resin. Heating the powder image then causes the resin to melt or fuse into or on the element. The powder is, therefore, caused to adhere permanently to the surface of the photoconductive layer. In other cases, a transfer of the electrostatic-charge image formed on the photoconductive layer can be made to a second support such as paper which would then become the final print after development and fusing. Techniques of the type indicated are well-known in the art and have been described in the literature, such as in RCA Review, vol. 15 (1954), pages 469-484.

The following examples are included for a further understanding of the invention.

EXAMPLE 1 A dye solution is formed by adding 0.45 gram of 2,6- diphenyl-4-(p-dimethylaminophenyl)thiapyrylium fluoroborate to a solvent mixture of 51 grams of methylene chloride and 34 grams of 1,1,2-trichloroethane. The mixture is stirred for 4 hours to facilitate dissolution. At the end of this time, 9.0'grams of Lexan 145 resin and 6.0 grams of 4,4'-diethylamino-2,2-dimethyltriphenylmethane photoconductor are added and the resultant combination is stirred for about 1 hour. Lexan 145 is a trademark for a Bisphenol A polycarbonate resin (General Electric Co.). After the polymer has been solubilized with the dye, the above formulation is separated into equal portions A, B and C and given the following treatment. Portion A is permitted to stand for 5 days without agitation at about -5 C. The formulation is then re-equilibrated at room temperature (about 25 C.) and stirred for 1 hour prior to coating. Portion B is permitted to stand for 5 days without agitation at room temperature (about 25 C.). The formulation is then stirred for 1 hour prior to coating. The final portion is coated immediately upon formation without further treatment. Each of the above formulations is coated at 0.003-inch knife setting onto a 0.4 optical density evaporated nickel conducting layer on a poly- (ethylene terephthalate) film support. After drying, the elements are crosssectioned and photomicrographs are prepared at 2500 magnification. Visual inspection of these photomicrographs indicates the aggregate size of formulation A is approximately 0.2 micron, while that of Samples B and C appears to be greater than about 1 micron.

EXAMPLE 2 The procedure of Example 1 is repeated to form an electrophotographic element from a coating dope which has been subjected to low-temperature conditioning and re-equilibrated at room temperature as with portion A of Example 1. The resultant element is charged in the dark and imagewise-exposed to form an electrostaticcharge pattern. This charge pattern is then developed using a developer comprised of black resin toner particles mixed with iron carrier particles. A good visible image results.

EXAMPLE 3 The procedure of Example 1 is repeated again to form three elements using 4-(4-dimethylaminophenyl) 2 (4- ethoxyphenyl)-6-phenylthiapyrylium perchlorate as the dye. These elements are prepared as in Example 1 and photomicrographs are made as before. The element pre pared from portion A which had been conditioned at low temperatures has an aggregate particle size of about 0.2 micron. This same element has a matte surface appearance. The other two elements which had not been so treated have an aggregate particle size of greater than about 1 micron and the surface of the elements appears matte. Similar results are obtained with 2,6-diphenyl-4-(p-dimethylaminophenyl)thiapyrylium hexafiuorophosphate.

EXAMPLE 4 The element prepared as in Example 3 using the dope which has been conditioned at reduced temperatures is charged, imagewise-exposed and developed as in Example 2. A good visible image results.

The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.

I claim.

1. A method of forming a heterogeneous photoconductive composition containing submicron-sized particles of a co-crystalline complex of dye and polymer, said method comprising the steps of first substantially completely dissolving at least one pyrylium dye in an organic coating solvent therefor, subsequently adding thereto an electrically insulating polymer containing an alkylidene diarylene moiety in the recurring unit, thoroughly mixing the combination of dye and polymer to form a coating dope containing from about /2 to about 30% by Weight of dye, cooling said dope to a temperature below about 5 C., maintaining said dope at this reduced temperature for a period of time such that a dry layer formed of said composition is heterogeneous, warming said dope gradually to coating temperature, forming a thin film about 10 to 1250 microns thick of said dope and drying said film to form a heterogeneous photoconductive composition having a continuous phase of polymer and a particulate discontinuous phase comprising submicron-sized particles of a co-crystalline complex of said dye and polymer.

2. The method-as described in claim 1 wherein said dye is selected from the groupiconsisting of th iapyrylium and pyrylium dye salts and said polymer is a carbonate polymer.

3. The method as described in claim 1 wherein said dope is maintained at a reduced temperature for at least about 24 hours without agitation.

4. The method as described in claim 1 wherein a photoconductor is added to the mixture of dye and polymer prior to cooling.

5. The method as described in claim 1 wherein the dye is selected from the group of compounds having the formula:

wherein: R and R are aryl radicals selected from the group consisting of phenyl and substituted phenyl radicals having at least one substituent selected from the group consisting of an alkyl radical of from 1 to 6 carbon atoms and an alkoxy radical of from 1 to ,6 carbon atoms; R is an alkylamino-substituted phenyl radical having from 1 to 6 carbon atoms in the alkyl moiety; X is selected from. the group consisting of sulfur and oxygen; and Z- is an anion.

6. The method as described in claim 1 wherein a mixture of two pyrylium dyes is used.

7. A method of forming a heterogeneous photoconductive composition containing submicron-sized particles of a co-crystalline complex of dye and polymer, said method comprising the steps of first substantially completely dissolving at least one dye selected from the group consisting of pyrylium and thiapyrylium dye salts in an organic coating solvent therefor, subsequently adding thereto an organic photoconductor and a carbonate polymer havingan alkylidene diarylene moiety in the recurring unit, thoroughly mixing the resultant combination of materials to form a coating dope containing from about 4/5 to about 30% by weight of dye, cooling said dope to a temperature below about 5 C., maintaining the dope at this reduced temperature for at least 24 hours,.warming said dope gradually to about 25 C., forming a thin film about 10 to 1250 microns thick of said dope and drying said film to form a heterogeneous photoconductive composition having a continuous phase of polymer and a particulate discontinuous phase comprising submicronsized particles of a co-crystalline complex of said dye and polymer.

8. The method as described in claim 7 wherein said particles are predominantly in the size range of about 0.01 'to about 0.75 micron.

9. The method as described in claim 7 wherein said dope is agitated as it is gradually warmed.

10. The method as described in claim 7 wherein said dope is cooled to a temperature between about 5 to about 196 C. and maintained at that temperature for a period of time between about 1 hour and about 15 days.

References Cited UNITED STATES PATENTS 3,052,540 9/1962 Greig 96-l.7 3,125,447 3/1964 Stewart 961.7 3,250,615 5/1966 Van Allan et al. 96l.7 3,488,705 1/ 1970 Fox et al. 96l.6 3,503,740 3/ 1970 De Selms 96-1.6 X

5 GEORGE F. LESMES, Primary Examiner M. B. WITTENBERG, Assistant Examiner U.S. Cl. X.R.

252--l; 260-327 TH, 345.1, 345.9, 37 PC, 34.2 

